I have a 1999 Ranger, 3.0. The engine is making a high pitch squealing noise and I already changed the idler pulley and the tensioner pulley, the noise went away for a bit after I changed the pulleys but came back.... any ideas on what it could be?

 

You need to make sure that all of the pulleys and pulley grooves are free and clear of any greasy substance, glazing, or anything else that might allow the belt to slip. Also make sure the belt isn't glazed over as well.

 

3.0l engine in known for Cam Synchronizer issues, bushings wear out and common noise for this is high pitch squealing, often described as similar to fan belt squealing.

Good read here: Ford Ranger 3.0L Camshaft Position Sensor

If you are not sure where the noise is coming from you can loosen the fan belt on COLD engine, remove it from crank shaft, and then start the engine, none of the pulleys are turning now.
If you hear the squealing then it is for sure Cam Synchro.
Don't run the engine more than 2-3 minutes without fan belt, you can REV the engine to see if squeal starts

 

Another thing to check, make sure you do not have anything leaking on the belt, like coolant or oil. The best thing to do is the belt trick that RonD said. This way u can isolate the noise and rule out pulls and belts too